Depending on your mtrbd. and it can by default, provide added "video ram" from system ram. It just does and it has to be in some default mode to do this. I found this out, once the mtrbd. is fully populated with ram it lends that extra ram to video. I see mine adding up to 1mb or maybe that's a total of 1mb. I had a 512mb video card and saw the extra ram reported in the video section of properties. Once you select better video display(defaults should kick-in) it tends to get from system ram to support better the video output. I found this out by accident and happy that it does this as I use now a 23in. monitor.
Here is a possible explanation of all this...
tally ho... -----Willy